a) Field of the Invention
The present invention relates to a zoom lens system designed for use with compact cameras and capable of photographing objects located at short distances.
b) Description of the Prior Art
In the recent years where cameras are designed so as to be fully automated, equipped with multiple functions and highly portable, zoom lens systems are generally adopted for these cameras for broadening photographing ranges.
In order to shorten total lengths of the zoom lens systems to be used with the cameras of this type, it is desirable to select the refractive power distribution which is similar to that of the telephoto type of lens systems, or the zoom lens systems can be made compact by selecting a paraxial refractive power distribution which locates the lens systems close to the image surfaces thereof.
As for focusing method for zooming lens systems, it is general to perform focusing by moving the first lens unit which can be controlled in the simplest manner.
When the first lens unit is selected as the focusing lens unit in a zoom lens system which covers a field angle corresponding to that of a wide-angle lens system, however, the front lens component has a large diameter and makes it difficult to remarkably shorten the distance as measured from the lens system to an object to be photographed with the lens system. This is because the focusing lens unit has a weak refractive power and the distance as measured to the entrance pupil is changed by focusing. Especially in a case of a zoom lens system which has a super wide field angle at the wide position thereof, the focusing method mentioned above does not permit shortening the photographing distance at which the zoom lens system can be focused on an object.
Further, as a zoom lens system which adopts the rear focusing method, i.e., performs focusing by moving the lens unit which has a negative refractive power and arranged on the most image side, there is known the zoom lens system disclosed by U.S. Pat. No. 4,701,033. However, this zoom lens system has a low vari-focal ratio. When an attempt is made to design this zoom lens system so as to have a higher vari-focal ratio and a more compact design, it will be difficult to focus this zoom lens system on an object. This is because an attempt to maintain the required amount of the marginal ray at the wide position of this zoom lens system will enlarge the diameter of the rear lens component, thereby enlarging the zoom lens system.
It is difficult to obtain a zoom lens system which can be focused on an object located at a very short distance and has a compact design when the first lens unit is selected as the focusing lens unit as in the general zoom lens systems as already described above.
Furthermore, there are known the inner focusing method and rear focusing method which select the lens units other than the first lens unit as focusing lens units. These focusing methods have advantages to permit remarkably shortening the distances to objects to be photographed and making zoom lens systems compact though the methods inevitably complicate the mechanism, software, etc. for controlling the focusing lens units. However, these focusing methods have a common defect that they allow the optical performance of the zoom lens systems to be degraded by focusing.